Ganga River witnessed unprecedented drying during 1991-2020 in the last 1,300 years

The Ganga River has been through unprecedented drying for 30 years between 1991 and 2020 in the last 1,300 years (700 to 2012). The drying seen during 1991-2020 is 76% more intense than the last severe drought that occurred in the 16th Century — between 1501 and 1530. The lastest 30-year period stands out as the driest period not only because of the number of drought years but also because the drought prolonged for a few years without a break

The Ganga River has been through unprecedented drying for 30 years between 1991 and 2020 in the last 1,300 years (700 to 2012), a study led by IIT Gandhinagar researchers and done in collaboration with the University of Arizona has found. The study was based on streamflow reconstructions from instrumental data, palaeohydrological records, and hydrological modelling. The results of the study have been published today in the journal of the Proceedings of the National Academy of Sciences.

The study led by Dr. Vimal Mishra, Professor at the Discipline of Civil Engineering, IIT Gandhinagar found frequent occurrence of droughts as one of the main reasons for the decline in mean streamflow during the recent period. The drying seen during 1991-2020 is 76% more intense than the last severe drought that occurred in the 16th Century — between 1501 and 1530.

While extreme wet years have generally exceeded extreme dry years in the Ganga River basin during the past 1,300 years, the trend appears reversed during the last three decades (1991-2020), wherein the number of extreme dry years increased “significantly”. While there was just one extreme wet year during the period 1991 to 2010 and only two extreme wet years between 2011 and 2020, the 30-year period between 1991 and 2020 stands out as the driest period of the Ganga River in the last 1,300 years.  

More number and prolonged duration of drought years

The reason why the 30-year period between 1991 and 2020 stands out as the driest period is not only because of the number of drought years but also because of the duration — the drought period prolonging for a few years without a break. “There have been several drought years and a few consecutive years of drought in the Ganga River basin during the period 1991-2020 when the river flow was less than the mean flow during the 1,300-year period,” says Dipesh Singh Chuphal, a PhD student at IIT Gandhinagar and the first author of the paper.

There were two seven-year drought periods (1991-1997 and 2004-2010), one five-year drought period between 2014 and 2018, and one four-year drought period between 1999 and 2002. Also, 2012 was a single-year drought year. Drought during 2004-2010 has been the most severe drought in the past 1,000 years, while drought during 1991-1997 has been the second worst drought period.

 “Multiple consecutive years of below-normal streamflow can often have a greater impact than a single year of severe drought,” he says. “The drought events are defined based on Runs theory.” The study found that severe and prolonged droughts have become more frequent in the last 30 years.

Driver of drought

Significant decrease in mean annual precipitation has been the main driver of drought in the last 30 years. Decrease of over 30% precipitation has been recorded in the western region, while there has been an increase by 10% in east-central India and Nepal. Decrease in monsoon precipitation not only reduces only reduces direct runoff, and, hence, reduced streamflow but also reduces groundwater storage. “Reduced groundwater recharge due to declining rainfall together with excessive groundwater extraction worsens streamflow reduction,” says Chuphal. “Time taken for groundwater recharge will be longer when drought prolongs for a few years.”

Drought years in the Ganga River basin were found to coincide with El Niño events in 1997, 2002, 2009, and 2015, which had caused widespread rainfall deficits across India. “However, even during the strong La Niña years of 2007 and 2010, the Ganga River basin received below-average precipitation,” he adds. “The below-average rainfall or drought even during La Niña years indicates the role of anthropogenic factors in driving Ganga River basin precipitation changes beyond natural interannual variability.”

“Due to climate warming, rainfall has to increase. But that is not the case in the Ganga River basin even during some La Niña years,” says Dr. Mishra. “How human activities and natural processes like monsoon variability interact is far more complex.”

Irrigation and aerosol loading are way higher in a part of central-north India. Studies by other teams and a 2012 paper in the Proceedings of the National Academy of Sciences by Dr. Mishra have pointed out the role of irrigation and high aerosol load in the atmosphere in reducing the temperature in the basin. As a result, the land-ocean thermal gradient is negatively affected, leading to reduced rainfall in the basin even as climate models predict increased rainfall with increasing temperature, Dr. Mishra says.

“The reason why climate models get it wrong is because human activities are not accurately represented in the models. Reality is far more complex than the expectation that anthropogenic warming will intensify rainfall,” Dr. Mishra adds.